This invention relates to a pickup for a digital disc used in practice as a videodisc, a digital audiodisc and the like, and particularly to a pickup in which elements thereof are integrated to form an integral assembly.
Recently, optical type videodisc systems and optical type videoaudio-disc systems have been put for practical use.
The pickup for an optical type digital disc of this kind have basically comprised a diode laser as a light source, and an optical system composed of various known individual optical parts such as a deflection prism splitter, a 1/4 wavelength plate, and an objective lens.
Basically, the optical system comprises an incidence route for condensing and illuminating a laser beam on a disc surface, and a reflection route for guiding a reflection light, whose intensity of light varies with a presence or absence of a pit of the disc, to light receiving elements. Usually, the incidence route and reflection route are designed to be identical with each other and directed oppositely, and the reflection light is separated towards the light receiving element by the action of the 1/4 wavelength plate and the deflection prism splitter. That is, a polarized surface of the reflection light, which has passed through the 1/4 wavelength plate and reached the deflection prism splitter, is rotated through 90 degrees with respect to a polarized surface of the projected light of a diode laser, and therefore, the reflection light is changed in course by the deflection prism splitter. Theoretically, the deflection prism splitter fully reflects the incident reflection light. There is no reflection light which is a return light to the diode laser.
However, for example, polycarbonate, acryl or the like used to form a transparent base plate of the disc has anistropy and dual reflective characteristic. Therefore, when an incident laser beam whose polarized surface is rotated through 45 degrees at the 1/4 wavelength plate is reflected at the surface of the disc, the polarized surface becomes rotated by the dual refraction. Such a reflection light again passes through the 1/4 wavelength plate and therefore, the polarized surface of the reflection light arrived at the deflection prism splitter is deviated through a rotated portion by said dual refraction from 90 degrees. That is, the reflection light arrived at the deflection prism splitter partly passes through and is formed into a return light to the diode laser. Such a phenomenon cannot be avoided as far as an optical system is employed in which the incidence route to the disc and reflection route are used in common and the reflection light is separated by the action of the 1/4 wavelength palte and deflection prism splitter.
Incidentally, as is known, the diode laser is variously changed in oscillating condition by the incidence of light from the outside. Change in oscillating condition of the diode laser is not preferable for the pickup. This means the deterioration of characteristic of regeneration, and in the digital audiodisc, deterioration in quality of sound often occurs. Of course, this problem can be solved by suitably inclining the incidence of laser light on the disc to vary the incidence route and reflection route. However, in the optical system using individual lens systems or a mirror as in prior arts, if the laser beam is made to be incident obliquely, the number of optical parts increases and optical parts of high density have to be integrated, and therefore, cost increases due to the increase in number of parts and high degree of part-integrating technique is required.
Further, an optical system have to provide a moving mechanism and driving mechanism or to control the focal position in a diametral direction of disc by the lens and control so that the laser beam may be illuminated with accuracy on the disc surface which involves shifting in the direction of thickness. This structure is small in size, and individual optical parts themselves and holders therefore are not compact. Thus, the pickup as a whole is inevitably of the considerably large device and the number of parts increases, resulting in high cost.